1. Field of the Invention
The present invention relates to: an active matrix substrate of a liquid crystal display device for use in the display section of a computer, a word processor, etc.; and a method for producing the same.
2. Description of the Related Art
Liquid crystal display devices including an active matrix substrate and a counter substrate with a liquid crystal layer interposed therebetween are known in the art. The active matrix substrate includes pixel electrodes provided in a matrix shape on a glass substrate, where scanning signal lines and data signal lines extend in the vicinity of the pixel electrodes so as to intersect one another. On the other hand, a counter electrode is formed on a surface of the counter substrate facing the liquid crystal layer. In a liquid crystal display device having the above configuration, potentials to be applied between the counter electrode and each pixel electrode is controlled so as to switch the orientation state of the liquid crystal molecules within each pixel region corresponding to the pixel electrode, whereby a display function is performed.
In such a liquid crystal display device, a light-shielding black matrix is typically provided on the counter substrate in order to improve the display quality of the device, the black matrix including openings having corresponding shapes to the pixel electrodes. Specifically, such a black matrix is provided to prevent light from leaking from the periphery of each pixel electrode. The black matrix is typically formed with margins to ensure that no light leaks from the periphery of each pixel electrode even if the black matrix is slightly misaligned with the counter substrate. However, the opening ratio of the liquid crystal display device decreases as the margins increase.
Moreover, in the field of liquid crystal display devices, it is commonplace to form a storage capacitance electrode in parallel to each pixel electrode for properly maintaining the potential of the pixel electrode. Since the storage capacitance electrodes must be formed in the pixel regions, they may also reduce the opening ratio of the display device.
One known type of liquid crystal display device is a projection type liquid crystal display device. There has always been a demand for reduction in size, increased definition and/or increased illuminance for this type of liquid crystal display devices. However, such demands have led to the following problems.
First, as the liquid crystal panels become smaller in size, the intensity of the light entering the liquid crystal panels is increased. The reason is that a smaller liquid crystal panel results in an increase in the ratio in size of the projected image to the liquid crystal panel (if the size of the displayed image remains the same), so that more intense light is required to display an image at the same luminance level than in the case of a larger liquid crystal panel. In general, the TFTs (thin film transistors) for controlling the state of the respective pixel electrodes are formed of thin silicon films, e.g., polycrystalline silicon. However, if light enters such a semiconductor layer, a photoelectric current is generated, thereby preventing the potentials of the pixel electrodes from being maintained at predetermined values; as a result, the display quality deteriorates. This problem becomes more prominent as the light intensity increases. In order to prevent this problem, it is necessary to form a light shielding film in each portion where a TFT is formed. However, such a light shielding film inevitably reduces the opening ratio.
Moreover, increasing the definition inevitably results in a decrease in the capacitance of each pixel electrode, thereby making it necessary to increase the physical size of the storage capacitance electrodes. However, increasing the size of the storage capacitance electrodes reduces the opening ratio.
Furthermore, in order to increase the illuminance of a liquid crystal panel, it is necessary to increase the intensity of the light entering the liquid crystal panel. This results in the need to form the above-mentioned light-shielding films, which inevitably reduce the opening ratio.
Therefore, in order to prevent a decrease in the opening ratio, it has been proposed in Japanese Laid-Open Patent Publication No.7-128685 to form a black matrix (light-shielding film) on an active matrix substrate, the black matrix being composed of a conductive light-shielding film, so that a capacitance created between the light-shielding film and each pixel electrode is utilized as a storage capacitance.
FIG. 5 is a plan view illustrating one pixel region of an active matrix substrate 200 proposed in Japanese Laid-Open Patent Publication No.7-128685. FIG. 6 is a cross-sectional view taken at line C-C' in FIG. 5. As shown in FIG. 5, the active matrix substrate 200 includes a plurality of pixel electrodes 213. Each pixel electrode 213 is formed in a region surrounded by scanning signal lines 208 and data signal lines 212 intersecting one another. A matrix of light-shielding film 202 (composed of a conductive film capable of shielding light) extends between adjoining ones of the thus-formed pixel electrodes 213 so as to partially overlap the pixel electrodes 213.
As shown in the cross-sectional view of FIG. 6, the light-shielding film 202 is provided above the data signal lines 212 and below an insulating film 214. A predetermined potential is applied to the light-shielding film 202 so that a region where the light-shielding film 202 overlaps the pixel electrode 213 via the insulating film 214 forms a storage capacitance. Moreover, a TFT 220 is formed by utilizing a portion of the scanning signal line 208 functioning as a gate electrode. The TFT 220 includes a source region 204 and a drain region provided on a transparent insulative substrate 201, the source region 204 and the drain region interposing the gate electrode.
In the above-described active matrix substrate 200, the light-shielding film 202, the data signal lines 212, and the like are essentially composed of a light-shielding material. These light-shielding films are all located above the TFT 220, which in turn is provided upon or above the transparent insulative substrate 201, so that the transparent insulative substrate 201, the TFT 220, and the light-shielding film 202 are layered in this order. However, this configuration makes the TFT 220 vulnerable to light entering through the transparent insulative substrate 201, thereby causing the above-mentioned deterioration of display quality. In addition, when the pixel pitch must be reduced in order to increase the definition of the display device, the storage capacitance must be increased for the above-mentioned reasons. However, since the storage capacitance is created by the overlapping portion between the light-shielding film and the pixel electrode, the storage capacitance can only be increased by increasing the size of the light-shielding film, thereby decreasing the opening ratio.